class Example { private static $count = 0; public static function setCount($value) { self::$count = $value; } public function getCount() { return self::$count; } } Example::setCount(3); $example1 = new Example(); echo $example1->getCount(); // output: 3 $example2 = new Example(); echo $example2->getCount(); // output: 3The 'self' keyword is part of the PHP language and does not require any external library or package.